High molecular weight linear thermoplastic polyesters, such as polymeric 1,4-butylene glycol terephthalates, have been widely used in films, fibers and, more recently, in moled articles because they crystallize very rapidly from the melt. The resins may also consist of mixtures of poly(1,4-butylene terepthalate) with minor amounts, e.g., from 0.5 to 2% by weight, of copolyesters or mixed esters having units derived from aliphatic or aromatic dicarboxylic acids other than terephthalic acid and/or aliphatic polyols, e.g., 1,2-and 1,3-glycols, other than 1,4-butanediol. All such polyesters can be made following the teachings of Whinfield, U.S. Pat. No. 2,485,319 and Pengilly, U.S. Pat. No. 3,047,539, suitably modified, if necessary. Poly(1,4-butylene terephthalate) is also commercially available.
When used in molding resins, the polyesters of the above type have outstanding surface gloss and high impact strengths and other physical properties.
In the present state of the art, it is known that the stiffness and impact strengths or such polyesters can be somewhat improved if they are blended with a vinyl aromatic polymer having a high aromatic unit content, prior to molding. It is specifically reported that these improvements can only be obtained if the aromatic units in the second resin component comprise greater than 50 wt.% and preferably 100% by weight of the monomer units in the second component. Although such compositions are also known to be improved in strength by adding reinforcements such as glass, the resistance to distortion by heat, and the impact properties are not as high as would be desirable.
In view of this state of the art, it has now unexpectedly been found that compositions comprising poly(1,4-butylene terephthalate) and a second resinous component in which the content of vinyl aromatic units is deliberately held below 50 wt.% and specifically in the range of 1 to 40 wt.% have superior physical properties, particularly retention of resistance to heat distortion. Moreover, the addition of such resinous components raises the already high impact strength of the polyester component. Moreover, this discovery carries through into such compositions which also are reinforced with materials such as glass fibers. In all cases the compositions process in a superior fashion during compounding and have outstanding moldability in conventional equipment.
The above results are particularly surprising because the prior art has repeatedly emphasized the need to insure that any second, resinous component to be used with a polyester resin, contains no less than 50% by weight of vinyl aromatic units.